The present invention generally relates to a grinding apparatus and more particularly, to a curved surface formation polishing apparatus for processing rotationally symmetrical objects such as optical lenses or mirrors, etc.
Commonly, in spherical lenses widely employed for optical systems, it is difficult to perfectly eliminate aberrations, and therefore, there has been an increasing demand for aspherical glass lenses capable of removing aberrations as performance of products is improved, with a tendency towards compact size and high density thereof. However, aspherical glass lenses have a problem that the mass production thereof is difficult, since they require a high degree of processing technique, and therefore it has been an essential object to establish a processing technique suitable for mass production at low cost.
In the conventional curved surface formation polishing apparatuses for rotationally symmetrical objects, particularly, in the aspherical lens formation grinding apparatuses, various attempts have been made as disclosed, for example, on pages 22-6 to 22-10, chapter 22 of "Trikepsue Ultra Precision Measuring Technique". One representative example of such aspherical lens formation grinding apparatus is described, for example, on pages 1717 to 1720, No. 7, VOL. 12 of "APPLIED OPTICS", and works on a processing principle as explained hereinbelow with reference to FIGS. 1(A) and 1(B) schematically illustrating construction of the known curved surface formation grinding apparatus in a top plan view (FIG. 1(A)) and a side elevational view thereof (FIG. 1(B)).
In FIGS. 1(A) and 1(B), the conventional aspherical lens formation grinding apparatus includes a rotary spindle 2 on which a processing tool 1 is mounted, a slide table 3 on which the rotary spindle is disposed so as to be controlled for displacement in directions indicated by arrows A, a workpiece rotating spindle 5 for rotatably supporting a workpiece 4, and a rotary table 6 on which said workpiece rotating spindle 5 is mounted so as to be pivotable in a direction shown by arrows B about a rotary shaft 7 of said rotary table 6. The forward edge of the processing tool 1 is rounded to have a predetermined radius of curvature as shown.
By the above arrangement, the prior art aspherical lens formation grinding apparatus functions in such a manner that, for creation of a predetermined aspherical cross sectional shape by polar coordinates, the position of the forward edge of the processing tool 1 with respect to the pivotal angle of the rotary table 6 or of the workpiece 4, i.e. the position thereof in the direction A is controlled, and thus, the grinding for the formation is effected by following the aspherical cross sectional shape in the radial direction of the workpiece 4.
In the known construction as described so far, however, since the direction of movement of the workpiece 4 by the rotation at the processing point is coincident with the grinding direction through rotation of the processing tool 1, processing loci of the abrasive grain cutting edge of said processing tool are formed in a spiral shape about a rotating center of the workpiece as the processing proceeds, with almost no crossing of each other, so that such spiral processing loci remain even after completion of the processing, thus resulting in deterioration in the quality of the processed surface, with a resultant surface roughness, for example, on the order of Rmax 0.05 micrometer to Rmax 0.2 micrometer.
Moreover, since it is difficult to impart a radius of curvature of high accuracy to the forward edge portion of the processing tool 1, while the control mechanism becomes complicated on the whole, owing to the requirement for the slide mechanism for the setting of the amount of processing, with control elements further required therefor, the allowance for the formed aspherical shape becomes .+-.1 micrometer or thereabout, and thus, it is difficult to achieve and maintain a sufficient accuracy in the configuration.
Furthermore, in the conventional arrangement as described above, for effecting finishing from the raw material, processing including more than two steps is required, and since the finishing is carried out by exchanging the processing tools, setting errors in the mounting of such processing tools take place, thus making it difficult to achieve a high processing accuracy.